Have your looked into Magicjack? I replaced my landline with it and am very satisfied.
It uses VOIP underneath the covers. It offers an iPhone app which can take calls (from the landline number) remotely. Texts, too. (I am not familiar with shortcode; I do not use shortcode.)
My guess is that you are not required to make use of the landline number as a landline after you set it up.